Hmmm … if what you want is a Shadow Catcher layer then do as follows –
1) Set the shadow casting layer’s Cast Shadow prop to Only
2) Set the floor/map area’s Accept Shadow prop to Only
3) Ensure that your comp’s background is set to full-on white
When you comp this Shadow Catcher layer, set its Blending Mode to Multiply – this BM will composite the layer with the white areas removed; leaving on the shadow. Take note that the Shadow Catcher layer is now a 2D layer. As such take note of where it is placed in the Timeline hierarchy if you have 3D interactions between/across other layers in the comp, since 2D layers will get in the way of 3D interactions involving layers below/above it in the Timeline hierarchy.
HTH
– Roland
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